Al-Qaida has moved to reassert its battered authority, appointing as its leader Ayman al-Zawahiri, the Egyptian militant mastermind who has vowed to avenge the death of Osama bin Laden with a September 11-style mass attack.

Zawahiri's succession was announced by al-Qaida's ruling council via the internet, six weeks after US special forces killed Bin Laden in a raid on a house in northern Pakistan.

It confirms the former surgeon, who has masterminded bombings including in Nairobi and New York over the past 13 years, as the world's most wanted man, with a $25m US bounty on his head.

The next most wanted figure is the Taliban leader Mullah Omar, with a $10m reward in the US for his capture.

Zawahiri will seek to leverage the notoriety to reinvigorate the militant franchise that has been marginalised by this year's Arab spring revolutions in the Middle East and weakened by the loss of Bin Laden, who is suspected by intelligence sources of having been in greater control of day-to-day operations than previously thought.

But large questions remain about whether Zawahiri, considered more argumentative and less charismatic than his Saudi predecessor, can unite al-Qaida's factions across south Asia and the Middle East while evading his American pursuers. Al-Qaida's goal of creating an Islamic caliphate spanning the Muslim world is limited to a pocket of lawless boltholes controlled by separate factions.

Yemen is now considered the home of al-Qaida's most powerful division, followed perhaps by Somalia, but the group remains based along the mountainous Afghanistan-Pakistan border.

Even before his appointment Zawahiri had taken on the mantle of a leader, adopting an inclusive tone intended to unite the various groups. "It was a complete change of language," said Baker Atyani, a Palestinian journalist and militancy expert, speaking of the latest video. "He is no longer giving instructions or going into detail but talking in general terms, like a leader, as Bin Laden used to."

From a family of wealthy doctors and religious scholars in Cairo – his grandfather was the grand imam of the al-Azhar university, the seat of Sunni learning – Zawahiri was a cheery if studious child, relatives said. He turned to radical Islam as a teenager, joining Egyptian Jihad in its struggle against the US-backed dictator Gamal Abdel Nasser.

He served three years in jail in the 1980s before going to Pakistan to fight against the Soviet occupation of Afghanistan. There, he joined forces with Bin Laden, and together they founded al-Qaida in Peshawar in 1988.

Zawahiri emerged as the "brains" of the militant group, playing a key role in several big attacks. The US has only indicted him for one: the August 1998 bombing of US embassies in Tanzania and Kenya, which killed 224 people and wounded several thousand. Since 2001 he has become the voice of al-Qaida, sending dozens of propaganda messages by video, audiotape and written text, spouting fiery rhetoric against his enemies.

"Pursuing the Americans and Jews is not an impossible task," he wrote in 2001. "Killing them is not impossible, whether by a bullet, a knife stab, a bomb or a strike with an iron bar."

Admiral Mike Mullen, chairman of the US joint chiefs of staff, made clear that Zawahri remained high on the U.S. list of hunted militants. "He and his organization still threaten us. And as we did seek to capture and kill - and succeed in killing - bin Laden, we certainly will do the same thing with Zawahri," Mullen told reporters in Washington.

Atyani, who met both men near Kandahar in June 2001, said: "Bin Laden was very quiet, he would count his words. Zawahiri likes to talk, to argue, to discuss."

Zawahiri's anger against the US is partly personal: his wife and two of his children were killed in the US invasion of Afghanistan following the 11 September attacks.

Thought to have been hiding in Pakistan's tribal belt, he escaped death during a drone strike in 2006 on a house in Bajaur, which killed 18 people. Other reliable reports over the past decade have placed him in the Waziristans.

But since the discovery, on 2 May this year, of Bin Laden – found in Abbottabad, a garrison town 35 miles north of Islamabad – experts believe Zawahiri could also be sheltering outside the tribal belt. "The border is too dangerous for anyone right now," said Ahmed Rashid, author of several books on militancy.

Zawahiri's other challenge is from inside al-Qaida. He lacks the popularity of Bin Laden, and is seen as a controlling micro-manager who lacks a popular touch. "Osama was much better at placating the various wings of al-Qaida – the Yemenis, Somalis, north Africans. I'm not sure Zawahiri can do that," said Rashid.

Efforts to capture Zawahiri may be hampered by growing distrust between Pakistan and the US. It has emerged that Pakistani intelligence had arrested five people accused of spying for the CIA in the operation to catch Bin Laden.

Pakistan's generals want to stop the CIA drone campaign in the tribal belt. Pakistan has reportedly stopped supplies entering the remote base where the drones are stationed, and the US has moved some across the border to Afghanistan.

And four of six intelligence "fusion centres", where the CIA and Pakistan's Inter-Services Intelligence share information, have been closed, according to the Washington Post.

Any further deterioration in relations between the spy agencies could seriously damage efforts against al-Qaida.In Egypt there was dismay at Zawahiri's role. "[He] seems even more of a madman than Osama was, and he will want to prove himself by going on the attack soon," Karim Sabet, a businessman, commented to Reuters.